Genesis of the Huangshannan high-Ni tenor magmatic sulfide deposit in the Eastern Tianshan, northwest China: Constraints from PGE geochemistry and Os–S isotopes

摘要

Graphical abstractDisplay OmittedHighlights?The sulfides of the ultramafic rocks from the Huangshannan deposit are characterized by high Ni tenor.?High Ni tenor is the result of olivine-sulfide-silicate liquid equilibrium at relatively high R values.?They were formed by mechanical transport and deposition of sulfide liquid in a magma pathway or conduit.?Two stage contamination model is indicated by the Os-S isotopes.?Pt and Pd depletion are attributed to sulfide liquid fractionation coupled with Cu-rich sulfide liquid percolation.AbstractThe Huangshannan magmatic Ni-Cu sulfide deposit is one of a group of Permian magmatic Ni-Cu deposits located in the southern Central Asian Orogenic belt in the Eastern Tianshan, northwest China. It is characterized by elevated Ni tenor (concentrations in recalculated 100% sulfide) in sulfide within ultramafic rocks (9–19wt%), with values much higher than other deposits in the region. Sulfides of the Huangshannan deposit are composed of pentlandite, chalcopyrite, and pyrrhotite and the host rock is relatively fresh, indicating that the high-Ni tenor is a primary magmatic feature rather than formed by alteration processes. It is shown that sulfides with high-Ni tenor can be generated by sulfide-olivine equilibrium at an oxygen fugacity of QFM +0.5, for magmas containing 450ppm Ni and 20% olivine. Ores with >10wt% sulfur have relatively low PGE and Ni tenors compared to other ores, R factor (mass ratio of silicate to sulfide liquid) modeling of Ni indicates that they formed at moderate R values (150–600). Based on this constraint on R values, ores with <10wt% sulfides in the Huangshannan deposit can be segregated from a similar parental magma with 0.05ppb Os, 0.023ppb Ir, and 0.5ppb Pd at R values between 600 and 3000. This, coupled with the supra-cotectic proportions of sulfide liquid to cumulus silicates in the Huangshannan ores imply mechanical transport and deposition of sulfide liquid in a magma pathway or conduit, in which sulfides must have interacted with large volumes of silicate magma. Platinum and Pd depletion relative to other platinum group elements (PGEs) are observed in fresh and sulfide-rich samples (S>4.5wt%). As sulfide-rich samples are also depleted in Cu, and as interstitial sulfides in those samples are physically interconnected at a scale of several cms, the low Pt and Pd anomalies are attributed to solid Pt and Pd phases crystallization and retention with the monosulfide solid solution (MSS) and Cu-rich sulfide liqu